Add AES-XEX mode to the default config file

Adding the AES-XEX mode to the default config file permits this mode to
be compiled, and tested, which may be needed for some programs in the
future.

include/mbedtls/config.h

@@ -501,6 +501,13 @@
  */
 #define MBEDTLS_CIPHER_MODE_CBC
 
+/**
+ * \def MBEDTLS_CIPHER_MODE_XEX
+ *
+ * Enable Xor-encrypt-xor mode (XEX) for symmetric ciphers.
+ */
+#define MBEDTLS_CIPHER_MODE_XEX
+
 /**
  * \def MBEDTLS_CIPHER_MODE_OFB
  *